1. Field of the Invention
The present invention relates to an ignition system for an internal combustion engine, and more particularly to an ignition system including an ignition coil assembly that comprises a primary coil and a secondary coil, the ignition coil assembly being disposed in a plug cap that covers a terminal of an ignition plug.
2. Description of the Related Art
Heretofore, there have widely been used ignition systems for internal combustion engines, the ignition systems including a high-tension cord connecting an output terminal of a secondary coil to an input terminal of an ignition plug. A noise suppressor resistor is connected either between the high-tension cord and the ignition plug terminal or in the ignition plug itself for suppressing electromagnetic noise that is induced by electric discharges between the central and ground electrodes of the ignition plug. The high-tension cord is required to be replaced periodically because of its limited dielectric strength and weather resistance.
Japanese Laid-Open Utility Model Publication No. 60-98288, published Jul. 4, 1985, discloses an ignition system that does not have such a high-tension cord. In the disclosed ignition system, an ignition plug is inserted in a plug insertion hole defined in the cylinder head of an engine, and has central and ground electrodes positioned in a combustion chamber of the engine. The ignition plug in the plug insertion hole has a proximal end covered with a plug cap that houses primary and secondary coils with a common core disposed therein. The secondary coil has an output terminal connected to the terminal of the ignition plug at its proximal end through a noise suppressor resistor. The ignition system of the above structure allows easy maintenance because it has no high-tension cord.
However, the common core of the proposed ignition system is small in diameter. Therefore, the magnetic field H generated by the primary coil and the magnetic fluxes B produced by the magnetic field H through the common core are related to each other such that, as shown in FIG. 4 of the accompanying drawings, the magnetic fluxes B reach a saturated region S even when the magnetic field H does not have a substantial magnitude. As a result, in the saturated region S, an increase .DELTA.Bp in the magnetic fluxes B is not proportional to an increase .DELTA.Hp in the magnetic field H produced under a predetermined voltage applied to the primary coil. Consequently, the voltage generated by the secondary coil is not sufficiently high, and the output voltage produced by the ignition coil assembly may be lowered.
In the disclosed ignition system, the cylinder head, which is made of metal, has a portion disposed around the primary and secondary coils in the plug cap. The magnetic fluxes produced by the primary coil induce an eddy current in the cylinder head portion, tending to generate magnetic fluxes in a direction to cancel out the magnetic fluxes generated by the primary coil. This phenomenon is also responsible for lowering the output voltage of the ignition coil assembly.
The present invention has been made to improve such an ignition system for an internal combustion engine which has no high-tension cord.